A radiotherapy planning apparatus uses a CT image of the inside of the subject body in radiotherapy planning. The radiotherapy planning apparatus calculates and decides radiotherapy parameters by using the CT values of the pixels of a CT image. More specifically, the radiotherapy planning apparatus calculates an electron density in the subject body from CT values, and calculates and decides radiotherapy parameters such as a radiation dose by using the electron density. The accuracy of an electron density is a very important factor in terms of guaranteeing the accuracy of radiotherapy parameters. The X-ray detection unit of an existing X-ray CT apparatus operates in the current integration mode of repeatedly integrating current pulses in the detection elements in accordance with an X-ray emission cycle. For this reason, acquired data includes noise components originating from an electronic circuit. That is, the noise components adversely affect a calculated electron density.
A radiotherapy system uses IGRT (image-guided radiotherapy). IGRT is a technique of accurately performing a radiotherapy while correcting the positional shift of a subject by using the X-ray image acquired in real time during a radiotherapy and the CT image acquired before the radiotherapy. A radiotherapy is performed over a relatively long period of time, and hence the subject moves sometimes. The motions of the subject include body motions such as the motions of the arms and legs and unavoidable motions accompanying physiological phenomena such as a respiratory motion. IGRT is equipped with an error detection technique of automatically stopping the emission of radiation upon detecting that a positional shift amount exceeds an allowable range. However, an existing imaging apparatus using X-rays operates in the current integration mode of repeatedly integrating current pulses in the detection elements in accordance with an X-ray emission cycle, and hence acquired data includes many noise components originating from an electronic circuit. The accuracy of error detection using X-ray images and CT images based on acquired data including noise components is not very high.
It is an object of an embodiment to provide a radiotherapy system and radiotherapy planning apparatus which aim at improving the accuracy of a radiotherapy.